Silver halide emulsions are generally prepared in the following steps: precipitation (with or without incorporated dopants), physical ripening (which may occur concurrently with the precipitation), desalting (also known as emulsion washing), and chemical ripening (often referred to as sensitization or emulsion finishing).
Emulsion precipitation is generally carried out in the presence of a peptizing agent, commonly gelatin, to keep the precipitated emulsion grains separated from one another and to avoid clumping or coagulation. The precipitation process consists of the chemical reaction of a soluble silver salt, usually the nitrate salt, with a soluble halide salt or mixture of halide salts to form a precipitate of the desired relatively insoluble silver halide salt or mixed salt.
Byproducts of this chemical reaction usually include soluble alkali nitrates, which are subsequently removed by the washing process, lest they form undesirable crystals when the emulsion is coated or otherwise used to make a photographic element.
Various other chemicals, sometimes even dyes, can be used during the precipitation process to prevent or minimize foaming, act as sensitizers, stabilizers, etc. In addition, certain chemicals may be added with the intention that they be incorporated into the silver halide crystal lattice for the purpose of controlling fog, reciprocity behavior, etc.
The chemical ripening or finishing process often consists of a chemical sensitization part and a spectral sensitization part, and these may be done consecutively or concurrently. Chemical sensitization commonly consists of treatment with sensitizing chemicals such as sulfur and/or gold compounds, followed by a heat treatment.
Spectral sensitization comprises the addition of a spectral sensitizing dye which is capable of being adsorbed to the emulsion grain surface and renders the emulsion sensitive to visible or infrared radiation, whereas the non-spectrally sensitized emulsion is sensitive only in the ultraviolet or blue regions of the electromagnetic spectrum. The spectral sensitizing dye may be present during the heat treatment or it may be added after the heat treatment.
In addition, other chemicals may be used in this process to act as modifiers, restrainers, antifoggants, stabilizers, etc.
It is frequently a goal of photographic research and development efforts to increase the sensitivity of a photographic element without any sacrifice, or even with an improvement, in other performance characteristics such as granularity, sharpness, reciprocity behavior, color reproduction, stability of the latent image, stability to temperature or humidity conditions, insensitivity to pressure effects, etc.
A common means of increasing photographic sensitivity is to use emulsions of larger grain size, but this generally leads to compromises with one or more of the characteristics noted above. Therefore, it is of extraordinary benefit if a means can be discovered of increasing the basic sensitivity of a photographic emulsion without altering its grain size. This essentially means increasing the efficiency of the emulsion in its conversion of the photons which fall upon it into developable latent image.
The use of heavy metal salts to increase emulsion sensitivity goes back at least as far as the work of Smith and Trivelli, U.S. Pat. No. 2,448,060. The desirable capability of some of these heavy metal salts to reduce the extent of reciprocity law failure has also been recognized for a long time.
Heavy metal salts, for example iridium, are most commonly incorporated into a photographic emulsion as a dopant during the silver halide grain forming process--prior to sensitization. A stable solution of K.sub.2 IrCl.sub.6 in nitric acid was disclosed by Leubner and White in U.S. Pat. No. 4,902,611. They disclosed that such a solution could be used either before or after the precipitation of a photographic emulsion.
More recently, as means other than heavy metal salts of increasing emulsion sensitivity have improved, it is now commonly observed that the use of these heavy metal salts in addition to the other sensitizers actually results in a loss of photographic sensitivity. Furthermore, the use of the heavy metal salts may lead to an undesirable contrast change in the characteristic density-log(exposure) curve of a photographic product.
In U.S. Pat. No. 4,693,965, Ihama and Tani sought to overcome the desensitizing effect of a dye by adding iridium to the chemical ripening along with a carbocyanine spectral sensitizing dye which forms J-aggregates.
In U.S. Pat. No. 4,810,626, Burgmaier et al disclosed the use of tetrasubstituted thiourea-type compounds as silver halide sensitizers. In the descriptions and examples which follow herein, it will be seen that it is not necessary for our purposes that the thiourea compound be tetra (i.e., fully) substituted. The preferred organic sulfur sensitizer of this invention is a mono-substituted thiourea compound.
There is a continuing need for photographic materials that will have improved latent image stability and reciprocity behavior without loss of speed.